The present invention relates to fluidized bed reactors in general and, more particularly, to a novel physical form for the inert heat storage particles of a fluidized bed of the type wherein a bed of fluidized fuel particles is established above and immediately adjacent to a static bed of inert heat storage particles.
Fluidized bed reactors made up of a single fluidized bed cell or a plurality of individual fluidized bed cells are well - known in the prior art. In such reactors, fuel particles are burned while maintained in a fluidized state most typically by the air supplied for combustion of the fuel particles. One advantage of burning fuel in a fluidized state lies in the ability of the bed of fluidized fuel particles to burn in a comparatively small volume, to conduct heat relatively rapidly to heating surfaces immersed within the fluidized particles, and to absorb sulfur in the fuel if the fluidized bed includes particles of a sulfur absorbent material as well as fuel particles.
The relatively rapid conduction of the heat of the fluidized bed to the heating surfaces immersed therein results from the high thermal conductivity that characterizes the fluidized mass of particles in the bed. Unfortunately, the high conductivity of the bed in the fluid state makes stable operation at low firing rates difficult. A small imbalance between the rate of heat liberation and the rate of heat removal can cause the bed temperature to fall by a relatively large amount. Such an imbalance, caused, by instance, by a mementary reduction in the fuel supply rate, can cause the bed temperature to fall below the ignition temperature of the fuel, particularly when operating at low loads since the bed temperature is then already relatively low. Since the ignition of the fuel particles in a fluidized bed cell is dependent predominately upon the temperature of the fluidized bed, the almost unavoidable heat flow imbalances in the system can cause the bed to become extinguished at low loads.
One solution to the aforementioned problem, as disclosed in U.S. Pat. No. 4,176,623, envisions a fluidized bed cell comprised of a bed of fluidized fuel particles positioned above and immediately adjacent to a static bed of inert heat storage particles. Fuel particles are fed to the static bed and ignited by the heat retained in the inert heat storage particles of the static bed. A supply of fluidizing combustion air is blown upwardly through the static bed into the fluidizing region in such a manner as to carry the fuel particles from the static bed into the fluidizing region and to fluidize the fuel particles within the fluidizing region while not fluidizing the inert heat storage particles forming the static bed.
One major problem associated with a fluidized bed employing a static ignition bed of heat storage particles as contemplated in U.S. Pat. No. 4,176,623, is the problem of maintaining sufficient void space between the inert heat storage particles of the static ignition. If sufficient void space is not present, the pressure drop encountered by the fluidizing air in traversing the bed may become excessive and result in the velocity of the air passing therethrough dropping below the minimum velocity necessary to ensure fluidization of the fuel particles within the fluidized bed above the static ignition bed. Additionally, an excessive pressure drop within the static ignition bed results in increased fan power requirements for the forced draft fans supplying fluidizing combustion air to the fluidized bed.
Additionally, if sufficient void space is not present, fuel particles injected into the bed from a central feed point will not be able to migrate across the bed. Therefore, good lateral fuel distribution, which is essential for efficient operation of the fluidized bed, will not be attained.